Known in the art are techniques whereby mechanical tests that generate load-penetration (P-h) curves are performed on a material to determine parameters that characterize the material, such as Young's modulus (E), yield strength (.sigma..sub.y), stress at 29% plastic strain (.sigma..sub.u), and hardness. See for example, WO 97/39333, published Oct. 23, 1997 and herein incorporated by reference, summarizing many known techniques in the Background section, and disclosing refinements thereof. However, though the above properties are important, and the known techniques are helpful in quickly characterizing materials, also of considerable importance is an understanding and evaluation of the preexisting stresses present in a material or structure.
Many common industrial processes create, typically, though not always, as an undesirable side effect, stresses in materials. For example, stresses are induced when cooling a material from a processing temperature; when depositing a coating or a thin film of a material on a substrate by any of a number of techniques, such as chemical vapor deposition (CVD), molecular beam epitaxy (MBE), thermal spray, sputtering, or evaporation; when shot peening, laser shot peening, bending or loading a material; when a material undergoes a phase transformation; and when welding materials together. The above represent just a few of the common industrial processes that can result in stresses being induced in the material in question.
Whether stresses are intentionally created or are, as noted above, an undesirable side effect, it almost always desired to quantitatively characterize them. As is appreciated by one of ordinary skill in the art, an understanding of these stresses can be important in determining likely failure modes, in assessing the cause of existing failures, in lifetime analyses, in quality control, and in a myriad of other applications. Based on a knowledge of preexisting stresses, process variables can be adjusted to optimize a particular process, and designs changed to avoid potentially damaging residual stresses. Understanding and evaluating such stresses can be important in assessing the integrity of structures having dimensions that range from a nanometer scale to a micrometer and to a macrometer scale.
For example, in the production of integrated circuits, electrically conductive vias are required to provide electrical communication between otherwise isolated circuit layers. To produce a via, the electrically conductive material is deposited in an etched hole. Failure of such vias is known to cause performance reductions or outright failure of integrated circuits, and ready evaluation of the stress in vias would be an important tool in screening defective chips or in optimizing deposition parameters to avoid the potentially damaging stresses. Yet no simple and economical technique is available that is reliable, fast and suitable for the physical scale of the vias (typically microns) and for use in a high volume production environment. Known techniques, such as hole drilling, are destructive, and can be particularly inappropriate.
Approaching the other end of the size scale, knowledge of the stresses can be of immeasurable benefit in determining, for example, the integrity of the numerous welds in the labyrinth of piping required in a nuclear power plant, and, on the macro scale, in assessing the integrity of submarine hulls.
Despite the importance of understanding and evaluating stresses, to the inventors' knowledge there exists no method or apparatus for quantitatively determining the preexisting stresses in a material based on an indentation test. Known methods, which include hole drilling, layer removal, strain, displacement or curvature measurements, X-ray diffraction or neutron diffraction, are typically tedious and/or expensive, and often not suitable for economically and quickly testing products, especially in the large volume production environment noted above. Many of the methods, such as hole drilling and layer removal, can render the test sample unsuitable for further use, and cannot be performed on any large number of samples. Unfortunately, known techniques can be more likely to be performed after, rather than before, a catastrophic failure, such as to determine why a tank car derailed, or why a turbine blade failed and destroyed an aircraft engine, and to assess liability of manufacturers.
Recent efforts, such as those of Tsui et al. (1996) and Bolshakov et al. (1996) are empirical observational studies that report no general or specific formulation for determining stress. The only major outcome of these studies is the realization that the overall hardness and the elastic modulus of an elastoplastic material may not be affected by any pre-existing elastic residual stress field. See Tsui, T. Y., Oliver, W. C. and Pharr, G. M (1996) "Influences of Stress on the Measurement of Mechanical Properties Using Nanoindentation: Part I. Experimental studies in an aluminum alloy", J. Mater. Res., vol. 11, pp. 752-759, herein incorporated by reference, and Bolshakov, A., Oliver, W. C. and Pharr, G. M (1996) Influences of Stress on the Measurement of Mechanical Properties Using Nanoindentation: Part II Finite Element Simulations," J. Mater. Res., vol. 11, pp. 752-759.
Accordingly, as improved techniques and apparatus for determining stresses would be a welcome advance in the art, it is an object of the present invention to address one or more of the foregoing disadvantages and drawbacks of the prior art.
It is a further object of the invention to provide methods and apparatus for allowing a simple mechanical test for determining the stresses in materials.
Other objects will be in part be apparent and in part appear hereinafter.